You can ask your local vet to prescribe the Fortaz after ensuring it's appropriate for the RI your snake has. Also your local vet can call around to exotic specialists to see if they'll do a phone consult to save you from driving.
That said, when my snakes have an RI the vet usually pushes Fortaz + Baytril or Fortaz + Naxcel to treat them.
Fortaz is also expensive and has a short shelf life so a lot of vets don't keep it on hand. A compounding pharmacy might be able to get it cheaper than the vet. Usually it's handed to us pre-measured in single-use syringes and frozen, we just thaw a syringe every three days.